This invention relates generally to the field of geophysical prospecting. More particularly, the invention is a method of identifying structural and stratigraphic discontinuities in a three-dimensional (3-D) seismic data volume containing dipping reflectors.
As part of the hydrocarbon exploration and production work process, geoscience interpreters often need to recognize and map subsurface structural features, such as faults, and stratigraphic features, such as channel- or sand-body edges, in three-dimensional seismic data. However, identifying structural and stratigraphic features in 3-D seismic data can be a time consuming, subjective, and difficult process. There is a need to generate, in a computationally efficient matter, a derivative data volume (i.e., a data volume derived from the original seismic data volume), that displays clear, sharply focussed structural and stratigraphic features that can be quickly recognized and exploited in the mapping process.
Several techniques have been used in the oil industry to enhance the interpretation of structural and stratigraphic features in 3-D seismic data. A well-known technique is to transform the original amplitude data into a coherence volume using a series of one-dimensional cross-correlation calculations. For every data sample in a volume, the cross-correlation calculation is performed using a user-defined vertical window with the equivalent portion of an adjacent trace. Typically, the vertical window is the target sample in question, plus 3-7 data samples above and below the target sample, depending upon the frequency of the data. This operation is repeated for all data samples and all traces, all in the same correlation direction. The correlation direction is generally in-line, cross-line, or either diagonal direction. The resulting coherence volume typically contains values normalized between xe2x88x921 and +1. For adjacent traces that are very similar, the value of the coherence sample will be close to +1, since +1 represents high correlation. This similarity, and hence correlation, is expected for adjacent traces that do not straddle a structural or stratigraphic discontinuity. For adjacent traces that do straddle a discontinuity, lack of similarity is expected. Thus, their coherence value would be closer to 0, since 0 represents no correlation. A coherence value of xe2x88x921 represents negative correlation, such as high correlation with phase reversal. Alternatively, coherence can be described with the inverse notion of discontinuity, in which low coherence equals high discontinuity and high coherence equals low discontinuity. In either case, this standard technique has limitations, though, because features perpendicular to the single correlation direction are highlighted, while features parallel to the correlation direction are poorly imaged.
Normally the cross-correlation calculations are conducted parallel to time slices. This direction of calculation can create problems if the seismic data volume contains reflections that dip significantly, because a calculation that is conducted parallel to a time slice searches across the dipping reflections. When a cross-correlation calculation searches across dipping reflections, it identifies poor correlation because it is comparing different parts of the seismic wavelet. It may then map low coherence or high discontinuity to the coherence volume, even where dipping reflections are continuous.
Dip-steering reorients the search in a cross-correlation calculation so that it is conducted parallel to dipping seismic reflections. Once dip-steering re-orients the search parallel to dipping reflections, the calculation compares the same parts of the seismic wavelet, and is able to correctly assign high correlation or low discontinuity to continuous reflections. If these reflections are cut by a discontinuity such as a fault or channel margin, this discontinuity is imaged much more clearly because of the contrast to the continuous reflections.
Prospective hydrocarbon reservoirs often have steep dips because they often are located at anticlines or inclined fault blocks. Dip-steering provides better images of discontinuities in these prospects. This enhances the ability to add reserves or make discoveries and produce complexly faulted traps. Easier and more efficient interpretation of complex fault networks should lead to cost reduction and performance improvement.
Bahorich and Farmer received U.S. Pat. No. 5,563,949, xe2x80x9cMethod of Seismic Signal Processing and Explorationxe2x80x9d, issued Oct. 8, 1996. This patent is commonly known as the xe2x80x9ccoherence cubexe2x80x9d patent. Bahorich and Farmer also obtained a continuation of this patent in U.S. Pat. No. 5,838,564, xe2x80x9cApparatus for Seismic Signal Processing and Explorationxe2x80x9d, issued Nov. 17, 1998.
Bahorich and Farmer""s ""949 patent describes a method for converting a fully processed 3-D seismic data volume into a cube of coherence measurements. According to their method, the 3-D data volume is divided into a plurality of horizontal slices, and each horizontal slice is further divided into a plurality of cells, each of which contains portions of at least three seismic data traces. As described in the ""949 patent, these at least three traces in each cell comprise a reference trace, an in-line trace, and a cross-line trace. The in-line trace and the cross-line trace are each compared to the reference trace in each cell using a measure of coherency. Then the in-line and cross-line coherency measures are combined to obtain a single value that is representative of the coherence of the three seismic traces for each cell. This process is repeated for every cell, using every trace in the 3-D seismic volume as a reference trace, in order to obtain a 3-D cube of coherence measurements. Bahorich and Farmer""s ""564 patent describes the corresponding apparatus for carrying out the process of their ""949 patent.
Bahorich and Farmer""s patented technique combines information from more than one correlation direction at each data sample in the 3-D seismic data volume, thereby highlighting structural and stratigraphic information along multiple azimuths. According to Bahorich and Farmer, in their invention xe2x80x9cthe concept of cross-correlation is extended to two dimensions by taking the geometric means between the classical one dimensional cross-correlationsxe2x80x9d (U.S. Pat. No. 5,563,949, column 4, lines 17-20). This technique has limitations, however. Combining information from different correlation directions may effect the image clarity of the structural and stratigraphic features. This decrease in clarity can make it more difficult to extract structural and stratigraphic information in automated mapping processes. In addition, the computational complexity of this procedure is significantly greater than the traditional method using classical one-dimensional cross-correlations. Further, Bahorich and Farmer""s ""949 and ""564 patents do not take into account the presence of reflection dip in the seismic data.
Higgs and Luo received U.S. Pat. No. 5,724,309 xe2x80x9cMethod for Geophysical Processing and Interpretation Using Instantaneous Phase and Its Derivatives and Their Derivativesxe2x80x9d, issued Mar. 3, 1998. Higgs and Luo""s ""309 patent describes a related technique for interpretation of faults and stratigraphic features. The technique uses instantaneous phase and its spatial derivatives to determine values of spatial frequency, instantaneous frequency, dip magnitude and dip azimuth. These values are plotted to produce a derivative seismic volume that highlights subsurface changes. The main advantage of this technique is its computational speed. However, the instantaneous phase and frequency images tend to be of lower resolution than traditional amplitude-derived cross-correlation images. A similar technique was also published by Hardage et al., 1998, xe2x80x9c3-D Instantaneous Frequency used as a Coherency/Continuity Parameter to Interpret Reservoir Compartment Boundaries Across an Area of Complex Turbidite Depositionxe2x80x9d, Geophysics, Vol. 63, No. 5, pp. 1520-1531. This technique uses instantaneous frequency images to define reservoir compartments by identifying facies boundaries. Neither of these two techniques discusses how to compensate for the presence of reflection dip in coherency calculations.
Gersztenkorn""s International Patent Application No. PCT/US97/00249, xe2x80x9cMethod and Apparatus for Seismic Signal Processingxe2x80x9d, was published as International Publication No. WO 97/39367 on Oct. 23, 1997. This technique generates a covariance matrix for an ensemble of seismic traces and then estimates the degree of similarity between traces by estimating the largest eigenvalue of the covariance matrix. It identifies the maximum coherence component and therefore identifies structural and stratigraphic discontinuities in the data at all azimuths. The main disadvantage is that because this technique estimates eigenvalues of the covariance matrix for each time sample in the volume, it is computationally intensive. This method discloses another form of continuity calculation, but it does not compensate for the presence of reflection dip.
Marfurt, Kirlin, Farmer, and Bahorich received U.S. Pat. No. 5,930,730 xe2x80x9cMethod and Apparatus for Seismic Signal Processing and Explorationxe2x80x9d, issued Jul. 27, 1999. The ""730 patent describes a method for identifying structural and stratigraphic features in three dimensions in the presence of reflection dip. After datumming is applied to remove a significant portion of the regional structural dip, a semblance calculation is applied as a function of time to multiple seismic traces in multiple directions to further estimate and correct for local dip. A maximum semblance cube is created that highlights structural and stratigraphic discontinuities, corrected for structural dips. Improved imaging is obtained in areas of higher structural dip and seismic noise. Unfortunately, this method is very computationally intensive.
Marfurt, Sudhaker, Gersztenkorn, Crawford, and Nissen have used a version of dip-steering for coherency calculations in their paper in Geophysics, Vol. 64, No. 1, pp. 104-111, January-February 1999, xe2x80x9cCoherency Calculations in the Presence of Structural Dipxe2x80x9d. The technique described in this publication examines the similarity of multiple traces at various time lags to estimate the dip of reflectors. The x and y components of apparent dip are estimated at each point in the seismic data cube using a xe2x80x9csemblance-basedxe2x80x9d algorithm. This algorithm calculates the semblance along various test dip/azimuth pairs and identifies the dip as that with the greatest calculated semblance. This estimate of dip is smoothed by either calculating its mean, median, or alpha-trimmed mean over a window approximately 10 times larger than the original window to obtain smooth apparent dip in the x and y directions. This step is intended to overcome the fact that some value of dip will be found across faults that does not correspond to the dip of the reflections on either side of the fault. After application of this filter, the adjacent reflection dips should dominate at the fault. These dip values are used to flatten the data and calculate coherency. An eigenvalue algorithm is used to calculate the similarity of traces in the locally averaged dip direction. The main advantage of this approach is the minimization of coherency artifacts due to the dip of reflectors and thus a sharpening of the image. However, this approach can be computationally intensive.
Sequence stratigraphy models the interplay of sedimentation, sea-level change, and subsidence in a geological setting. P. R. Vail et al. have described the importance of seismic sequence analysis to structural and stratigraphic interpretation in their paper xe2x80x9cSeismic Stratigraphy and Global Change of Sea-levelxe2x80x9d, AAPG Memoir 26, 1977. In stratigraphic interpretation, reflection patterns are the clues used to reconstruct the depositional environment. During the process of determining the depositional sequence, geologists often use seismic data to interpret stratigraphic horizons.
Pattern recognition and image processing have also been applied to automatic tracking of seismic horizons. Y. C. Cheng and S. Y. Lu have described a procedure called seismic skeletonization in their paper xe2x80x9cThe Binary Consistency Checking Scheme and its Applications to Seismic Horizon Detectionxe2x80x9d, IEEE Transaction on Pattern Analysis and Machine Intelligence, Vol. II, No 4, 1989. In that paper, a correlation based on the similarity of seismic reflectors in the adjacent traces was used to extend the reflectors laterally. This procedure then fits straight-line segments to the continuous connected reflectors. The procedure goes further to employ a sorting process on the straight-line segments so that the bedding sequence of seismic reflectors would follow the rule of superposition in which younger beds lie on top of the older beds. The seismic skeletonization procedure also uses an iterative approach. Strong reflectors are tracked first, and then weak reflectors are tracked at the later iterations. The geological trend is thus incorporated into the result.
It can be seen from the foregoing that a need exists for a computationally efficient and accurate method for identifying structural and stratigraphic features in 3-D seismic data in the presence of reflection dip.
The present invention is a method for detecting structural and stratigraphic discontinuities in a volume of seismic data samples in the presence of reflection dip. A primary and at least one secondary direction are selected in the volume of seismic data samples. Skeleton patches are identified in the volume of seismic data samples in the primary and secondary directions. The skeleton patches comprise groups of connected seismic samples representing seismic horizons in the volume of seismic data samples. Apparent dip is calculated in the primary and secondary directions at each data sample within these identified skeleton patches. This creates a primary direction dip volume and at least one secondary direction dip volume, respectively, at corresponding data sample locations. A filter is applied to the primary and secondary direction dip volumes to fill in values at data sample locations not within the identified groups of skeleton patches. Finally, a discontinuity volume is determined from calculated one-dimensional, two-trace discontinuity values in the primary and secondary directions. The calculated apparent dips at corresponding data sample locations from the primary and secondary direction dip volumes determine which portions of the data samples to use in the corresponding discontinuity value calculations.